232 GASTEROPODA AND PTEROPODA. 



which bring food to the mouth. It persists in many forms for a 

 very long period. Within the area of the velum there appear the 

 tentacles and eyes (fig. 103 B). The latter are usually formed at 

 the base of the tentacles. 



The foot grows in most forms to a very considerable size. 

 On its hinder and dorsal surface is formed the operculum as a 

 chitinos plate which originates in a depression lined by thick- 

 ened epiblast, much in the same way as the shell (fig. 103 B and 

 C, op}. In the typical larval forms it is only possible to distin- 

 guish the anterior flattened surface of the foot for locomotion and 

 the posterior opercular region, but special modifications of the 

 foot are found in the Pteropods and Heteropods, which are 

 described with those groups. The foot very often becomes richly 

 ciliated, and otic vesicles are early developed in it (fig. 101, of). 



All the Gasteropods and Pteropods have a shell-bearing 

 larval form like that first described, with the exception of a few 

 forms, such as Umax and perhaps some other Pulmonata, in 

 which the shell-gland closes up and gives rise to an internal 

 shell. 



The subsequent metamorphosis in the different groups is very 

 various, but in all cases it is accompanied by the disappearance 

 of the velum, though in some cases remnants of the velum may 

 persist as the subtentacular lobes (Lymnaeus, Lankester) or the 

 lip tentacles (Tergipes, Nordmami). In prosobranchiate Gaste- 

 ropods the larval shell is gradually added to, and frequently 

 replaced by, a permanent shell, though the free-swimming velig- 

 erous larva may have a long existence. In many of the Opis- 

 thobranchiata the larval shell is lost in the adult and in others 

 reduced. Lankester, who has especially worked at the early 

 stages of this group, has shewn that the larvae are in almost 

 every respect identical with those of prosobranchiate Gastero- 

 pods. They are all provided with a subnautiloid shell, an oper- 

 culated foot, etc. The metamorphosis has unfortunately been 

 satisfactorily observed in but few instances. In Heteropods and 

 Pteropods the embryonic shell is in many cases lost in the adult. 



The following sections contain a special account of the develop- 

 ment in the various groups of Gasteropoda and Pteropoda 

 which will complete the necessarily sketchy account of the pre- 

 ceding pages. 



